The present invention relates to separation of ions by liquid chromatography followed by ion suppression and detection.
One form of ion chromatography includes the use of two ion-exchange columns in series, followed by a flow-through detector. The first column separates the ions of injected sample by eluting of the sample through the column using an electrolyte eluent. In the second column, termed a "suppressor", the electrical conductivity of the electrolyte in the eluent, but not of the separated ionic species, is suppressed so that the ionic species can be determined by a conductivity cell. This technique is described, for example, in U.S. Pat. Nos. 3,897,213; 3,920,397; 3,925,019; and 3,926,559. The disclosures of such patents are incorporated herein by reference.
An improved form of suppressor has been developed in the form of a membrane, such as a fiber, which allows for continuous analysis of consecutive samples without periodic regeneration or replacement of the suppressor column. One such column is described in published European patent application No. 32,770, in which such a charged fiber membrane is used in place of the resin bed. The sample and eluent are passed through the fiber with a flowing regenerant at the outside wall of the fiber. The fiber comprises an ion-exchange membrane partitioning the regenerant from the effluent of chromatographic separation. The membrane passes ions of the same charge as the exchangeable ions of the membrane to convert the electrolyte of the eluent to weakly ionized form, followed by detection of the separated ions. For analysis of cations, an eluent, including a strong base electrolyte, such as sodium hydroxide, is used for separation. A dilute aqueous solution of a strong acid, such as sulfuric acid, is used as the regenerant. The ion-exchange membrane is of a form which allows sodium ion to permeate out of the fiber while hydrogen ion permeates in. In this manner, the sodium hydroxide in the eluent stream is converted to de-ionized water and the sodium ions are dispersed in the regenerant and ultimately to waste as sodium sulfate and sodium bisulfate. The cations to be analyzed are in hydroxide form and of suitable ionization for detection by a conductivity detector or the like. For anion analysis, an appropriate modification of the charges of the components are made. As used herein, the term "prior art membrane suppression" refers to a system of this general type.
Prior art membrane suppression is highly effective for suppression in a system where the ions to be analyzed are highly ionized in the acid or base form, the form in which they are passed to the detector. However, if such ions are weakly dissociated in the acid or base form, difficulties arise in the detection of the ion conductivity of such analyte ions based upon their conductivity. Thus, the system may not be effective for analyzing weakly basic (herein "weak") cations such as amines or weakly acidic (herein "weak") anions such as carboxylates.
One technique for the separation of weak anions or cations is by ion exclusion chromatography (IEC) using a medium, typically a resin bed, with permanently attached ion-exchange sites, (hereinafter "IEC"). The IEC mode of separation is described in U.S. Pat. No. 4,314,823, incorporated herein by reference. Briefly summarized, an accepted theory of separation by ion exclusion chromatography is that the resin network serves as a boundary, which behaves as a semi-permeable membrane between the interstitial liquid in the resin particles and the occluded liquid inside the resin. Due to Donnan exclusion, highly ionized molecules, such as strong mineral acids or bases, are excluded from the resin particles and pass directly through the column in the void volume peak. Weakly ionic molecules may enter the resin phase in acid or base form, depending upon the form of the ion-exchange sites, and are retained by the resin for a later elution than the strong acids. In general, referring to the separation of anions, the weak acids in their molecular substantially non-ionized form can penetrate into the interior of the ion-exchange resin while the highly ionized acids are excluded. By using ion-exchange resin in hydrogen ion form, salts of weak acids which are highly ionized (e.g. of alkali metals) are converted to their acid form which may be retained by the column. For example, sodium acetate is converted on the column to acetic acid and the sodium ion is retained by the column. Thereafter the acetic acid is resolved from other weak acids and eluted from the column in a separate peak volume which can be detected. The above dicussion of ion exclusion chromatography applies in analogous manner to the separation of cations, except, in this case, an ion-exchange resin in the hydroxide form is used in the ion exclusion column.
Another system ffr separating and analyzing ions is by substituting mobile phase ifn chromatography (MPIC) for the ion chromatography, using a resin bed suppressor. This MPIC system is described in U.S. Pat. No. 4,265,634. Here, the separation medium is a porous hydrophobic chromatographic resin with essentially no permanently attached ion-exchange sites. An ion-exchange site-forming compound, including a hydrophobic portion and an ion-exchange site, is passed through the column and forms reversible adsorptive bonds with the resin to create ion-exchange sites.
It is an object of the invention to provide a system of ion analysis combining certain prior art principles utilizing membrane suppression but which is an improvement upon prior art membrane suppression, particularly for the analysis of ions which are only weakly associated in their acid or base form.